Both AC and DC tungsten-inert gas (tig) welding devices are generally known in the art. In such devices, a hollow housing is provided with a tungsten electrode positioned therein. An inert gas such as argon is expelled under pressure from the open front end of the housing. The inert gas surrounds the tungsten electrode as well as the region of metal to be welded.
It has also been known, with respect to tig welding, that it is necessary to supply a high voltage, on the order of 3500 volts or so to break down the gas and start the arc. Conventionally, a spark gap oscillator has been used in the prior art for this purpose. The spark gap oscillator will generate a 3500 volt, relatively high frequency, output signal which can be used to ionize the inert gas and thereby start the arc.
Conventionally, in prior art systems, the spark gap oscillator has been powered by 60 Hz line voltage through a step-up transformer to generate a 60 Hz high voltage input signal, on the order of 3500 volts, which is then used to power the spark gap oscillator. However, the same 60 Hz signals are also used to provide output voltage and current to the tungsten electrode and the metal members being welded.
As a result, the high voltage input to the spark gap oscillator can be in phase with the welder output voltage and current.
In addition to being used to initiate the arc, in AC tig welding equipment, the output of spark gap oscillator is critical to maintaining the ionized condition of the inert gas during time intervals when the output voltage is going through zero and changing polarity. Without the high frequency signal from the spark gap oscillator during these transition periods, the arc will become extinguished.
However, the input AC voltage has to exceed a predetermined threshold before the spark gap oscillator receives a high enough voltage to result in the air gaps breaking down to produce the desired oscillation. There is thus a dead zone when the input AC voltage to the spark gap oscillator is below this predetermined threshold. This dead zone will occur during the same time intervals that the output voltage is also crossing through zero, provided the voltage to the spark gap oscillator is in phase with the output voltage. Hence, the time when the output voltage should be receiving the high frequency signal from the spark gap oscillator so as to maintain the inert gas in its ionized condition is exactly the time when the spark gap oscillator will cease to function.
To avoid this problem, it has been known generally to shift the phase between the output AC voltage and the AC voltage input to the spark gap oscillator. This solution has had some measure of success. However, newer tig welding machines with wave balance controls have varying points at which the output voltage transitions through zero due to the output current and the wave balance setting.
In such machines a phase shifting circuit does not solve the problem completely. Due to the asymmetrical shape of the output wave form it may not be possible to adjust the phase of the spark gap oscillator input voltage so that the oscillator is functioning at all times that output transitions occur. Hence, the results of this solution tend to be inadequate.
Thus, there continues to be a need to be able to drive the spark gap oscillator so as to minimize or completely eliminate dead zones in the output thereof. Further, it would be desirable to be able to not only incorporate such driving circuitry into new machines but to be to able to retrofit existing tig welding units so as to eliminate this dead zone.